Ignore OpenMP on machine that does not have it

Question

I have a C++ program using OpenMP, which will run on several machines that may have or not have OpenMP installed.

How could I make my program know if a machine has no OpenMP and ignore those #include <omp.h>, OpenMP directives (like #pragma omp parallel ...) and/or library functions (like tid = omp_get_thread_num();) ?

Solution 1

OpenMP is a compiler runtime thing and not a platform thing.

ie. If you compile your app using Visual Studio 2005 or higher, then you always have OpenMP available as the runtime supports it. (and if the end-user doesn't have the Visual Studio C runtime installed, then your app won't work at all).

So, you don't need to worry, if you can use it, it will always be there just like functions such as strcmp. To make sure they have the CRT, then you can install the visual studio redistributable.

edit:

ok, but GCC 4.1 will not be able to compile your openMP app, so the issue is not the target machine, but the target compiler. As all compilers have pre-defined macros giving their version, wrap your OpenMP calls with #ifdef blocks. for example, GCC uses 3 macros to identify the compiler version, __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__

Solution 2

OpenMP compilation adds the preprocessor definition "_OPENMP", so you can do:

#if defined(_OPENMP)
   #pragma omp ...
#endif

For some examples, see http://bisqwit.iki.fi/story/howto/openmp/#Discussion and the code which follows.

Solution 3

Compilers are supposed to ignore #pragma directives they don't understand; that's the whole point of the syntax. And the functions defined in openmp.h have simple well-defined meanings on a non-parallel system -- in particular, the header file will check for whether the compiler defines ENABLE_OPENMP and, if it's not enabled, provide the right fallbacks.

So, all you need is a copy of openmp.h to link to. Here's one: http://cms.mcc.uiuc.edu/qmcdev/docs/html/OpenMP_8h-source.html .

The relevant portion of the code, though, is just this:

#if defined(ENABLE_OPENMP)
#include <omp.h>
#else
typedef int omp_int_t;
inline omp_int_t omp_get_thread_num() { return 0;}
inline omp_int_t omp_get_max_threads() { return 1;}
#endif

At worst, you can just take those three lines and put them in a dummy openmp.h file, and use that. The rest will just work.

Comments

[Tim]

My problem is that I want to run the program without multi-threading on those machines that don't have it. GCC below version 4.2.x doesn't support OpenMP. So I want to make my Makefile be able to tell this and ask g++ to ignore the OpenMP part in my program instead of failing the compilation. Any idea?

[YvesgereY]

It's unnecessary to protect #pragma omp, since such pragma is not intrusive (safely skipped when openmd is not activated).

[YvesgereY]

+1 for #pragma semantic. -1 for ENABLE_OPENMP, which is not standard compliant. Use _OPENMP instead.

[Andrew Dalke]

While true, if one likes to compile without warnings even under -Wall then a compiler message like "warning: unknown pragma ignored" will be annoying.

[johan d]

About compilation warning, I use to compile with -Wno-unknown-pragmas for this very reason. Problem: may affect other pragmas in the project and remove useful warnings.

[Jan Hudec]

-1: Compiler version is completely inadequate. Both compiler and runtime have to support openmp and it has to be enabled too. The same compiler version may support openmp on one computer and not support it on another.

[jww]

"... the header file will check for whether the compiler defines ENABLE_OPENMP" - I don't think this is true. Could you provide a reference for ENABLE_OPENMP? I could not find it in the preprocessor defines. See What preprocessor define does -fopenmp provide?

[Arne]

I am compiling with clang 3.6.2 and clang++ -fopenmp tells me _OPENMP is not defined.

[KRoy]

This if-block is useful to hide function call like omp_set_num_threads() or omp_get_thread_num() . #if defined(_OPENMP) tid = omp_get_thread_num(); #else tid = 0; #endif

[Mikhail T.]

LLVM's page on the subject seems to suggest, OpenMP was only properly introduced into the compiler-suit with version 3.8. The earlier versions (trying 3.6 and 3.7 on FreeBSD here) seem to accept the -fopenmp flag, but ignore it... With clang-3.8 the _OPENMP is defined and the parallelization actually works.

[emprice]

This is also useful if using OpenMP locks (for example) depending on whether running in parallel or not. Those are not no-ops if OpenMP is not being used (say, with -fno-openmp).

[jww]

@YvesgereY - Re: guarding use of pragmas... GCC is OpenMP compliant but issues unknown pragma warnings if OpenMP is not enabled. It breaks a clean compile that many use as a security gate. Also see Issue 66943, GCC warns of Unknown Pragma for OpenMP, even though it support it.

[tim18]

As the target execution platform will need to have installed library support for the compiler used to build the application, it shouldn't be necessary to build on each machine. In the example just above, pragma omp simd by itself isn't likely to depend on library support. It should be enough to write

[tim18]

#pragma omp simd if(_OPENMP >= 201307) without multiple copies of identical source code.

[jww]

@tim18 - "... pragma omp simd by itself isn't likely to depend on library support" - The use case I was thinking about was a distro model. Building on an i686 machine, and then running on another i686 machine without SSE2 and friends. SSE2 and friends is part of the core instruction set on x86_64, but it is optional on 32-bit processors. This can work in theory using runtime dispatching but I don't see where the prerequisites/requirements are in place so that is does work in practice.

[jww]

@tim18 - Have you confirmed #pragma omp simd if(_OPENMP >= 201307) works on Visual Studio with its ancient 2002 OpenMP implementation? When I attempt to use it with Visual Studio 2015 I receive test.cpp(8): error C3001: 'simd': expected an OpenMP directive name

[Royi]

Does the _OPENMP macro defined on all major x86 compilers (MSVC, Clang, GCC)?

[Jake Lishman]

If _OPENMP is not defined, then the #pragma omp won't be understood anyway so the if will have no effect in this case. You also say there are valid reasons not to use #pragma, but your linked article is about #pragma once (not #pragma omp which is totally different) and your solution uses #pragma omp anwyay. It is not possible to use OpenMP functionality without #pragma omp.